(a) Field of the invention
The present invention relates to a method for manufacturing a poly-V pulley of metal plate having a series of V-grooves formed around the outer circumferance of an annular flange wall thereof with a predetermined pitch between grooves.
(b) Brief description of the prior art
Various kinds of poly-V pulleys have been developed to be used for poly-V belts recently developed instead of V belts.
U.S. Pat. No. 3,977,264 discloses a technique of manufacturing a poly-V pulley using a metal plate blank. It can be summarized as follows. After a one-piece cup-shaped metal blank is pre-molded to form a series of corrugation-like valleys around the outer circumference of an annular flange wall adjacent to the open end thereof, the flange wall is axially compressed, and then a rotary forming roller is pressed into each of the valleys to form predetermined V-grooves around the outer circumference thereof.
The manufacturing technique disclosed by the abovementioned U.S. patent causes a problem particularly in the squeezing and compressing process of the flange wall. Though the U.S. patent discloses a step in which the flange wall is compressed by upper and lower pressing forms keeping inner and outer surfaces of the flange wall enclosed by fixed wall faces of inner and outer forms, it is difficult for this step to allow the flange wall to be evenly or regularly folded in zigzag manner. Namely, though the flange wall has been pre-molded to have a corrugated cross section, it is seldom seen that crests and valleys of the corrugation are evenly or regularly bent by the compression due to upper and lower pressing forms, but they are unevenly or irregularly bent in most cases. Therefore, the pitch between valleys formed on the outer circumference of the flange wall becomes irregular, thus making the subsequent rotary roller pressing operation difficult. In addition, the uneven or irregular bending of the corrugation causes crests and valleys thereof to be excessively pressed against fixed wall faces of inner and outer forms enclosing the flange wall, thus making it difficult to pull off the compressed corrugated cup-shaped blank from forms.